This invention relates to an electrode and a method for stimulating tissue in medical applications, and more particularly to an implantable cardiac defibrillation electrode together with associated electronics and a method for performing cardiac defibrillation.
Electrodes implanted in the body for electrical stimulation are well known. More specifically, electrodes implanted on or about the heart have been used to reverse (i.e., defibrillate or cardiovert) certain life-threatening cardiac arrhythmias, by applying electrical energy to the heart via these electrodes to return the heart to normal sinus rhythm. The amount of energy delivered to the heart during defibrillation (or cardioversion) depends on the placement of the electrodes on or about the heart and the ability of the electrodes to distribute the energy uniformly through the heart.
Prior devices for efficiently delivering defibrillation waveforms from electrodes to heart tissue also are known. See for example, commonly assigned U.S. Pat. No. 4,768,512. In this prior device, a truncated exponential defibrillation pulse is chopped into a plurality of consecutive pulse segments and delivered to the heart via an electrode pair. Such high frequency waveforms compensate for the various frequency-dependent impedances throughout the heart tissue to distribute energy more effectively.
The present invention is based upon the recognition that the high energy delivered to a fibrillating heart during defibrillation causes at ionic current to develop at the electrodes. The conversion from an electric current to an ionic current produces gas at the electrode-tissue interface which acts as an insulator between the electrode and the tissue to which the defibrillating energy is being delivered. As a result, the amount of electrical energy actually delivered to the tissue from the electrode is reduced, and therefore, some of the defibrillating electric field developed between the electrodes never effectively reaches the heart. Accordingly, there is a need to increase the ability of defibrillation electrodes to deliver energy to a fibrillating heart.
By increasing the efficiency of the transfer of energy from the electrodes to the heart, the amount of energy required at as can be reduced. As a result, the size the input of the electrodes can be reduced. As a result, the size of the unit containing the defibrillation/cardioversion circuitry can be reduced, or the life of the unit can be correspondingly increased.